Gas-phase polymerization is a technique for preparing polyethylene, which can be carried out in a fluidized or stirred bed reactor in the presence of a catalyst, ethylene, fluidization gas and a molecular weight regulator. In some processes, the molecular weight regulator is hydrogen.
For a catalyst system, gas-phase ethylene polymerization activity can depend on the polymerization conditions, such as temperature and pressure. In some instances and when the activity is not satisfactory, the amount of catalyst fed to the reactor is increased or the residence time of the reactor is increased.
Titanium based Ziegler-Natta catalysts can be used for the gas-phase polymerization of ethylene in combination with aluminum alkyls. When hydrogen is used as molecular weight regulator, a fraction of ethylene is reduced to ethane thereby lowering the amount of ethylene available for polymerization. Moreover, as during the continuous polymerization ethane accumulates in the ethylene, removal of the ethane becomes necessary and can be achieved by distillation or by purging. The problem of ethane production is pronounced when producing ethylene polymers having relatively low molecular weight.